I've yet to play in myth (Im at 68 with barely an raids going when I'm on
) and I don't PvP, but Shiny shung provides some good damage when it is otherwise unsafe to transform or if all of them are on cooldown. Lime can't do much outside transforms, and he has no other projectiles.
IMO It truly shines in Kirin/Lower/Upper raids where the bosses are weak to it and it is a good "spit-and-run" attack in addition to RB sticky Jelly, Though in Myth, the damage is probably kinda ew in comparison to transforms (unless they are weak to cold... I'm unaware of myth weakness/resistances). I personally prefer the rebirth one, but you didn't ask about that one
Hopefully a myth-er Lime can shed more light on your question though